Semiconductor lasers are increasingly being used in a variety of applications. Often the output signals of these lasers have an unwanted broad spectral line width; in many applications it is necessary to have light of a higher spectral purity, as well as a laser having a stabilized and controlled oscillation center frequency. These constraints have stimulated a plethora of semiconductor laser frequency stabilization techniques.
With the increasing availability of semiconductor lasers of different wavelengths, diode lasers have become very important tools for spectroscopic applications. However, a major disadvantage of diode lasers is their large spectral linewidth, typically tens of megahertz in the free running case. A standard approach to improving the spectral purity of diode lasers has been to use the effect of optical self-locking of the laser to an external Fabry-Perot cavity. In a typical arrangement, an external cavity acts as a frequency selective mirror reflecting light back into a laser if the laser frequency is close to a cavity resonance. A strong sensitivity of the laser output to back reflections normally leads to chaotic behaviour of the laser frequency, but if the laser diode current and the intensity and the phase of the light reflected back into the laser are adjusted properly, the effect of optical self-locking of the laser to the Fabry-Perot cavity appears. In this case, the spectral linewidth is typically reduced by more than two orders of magnitude relative to that observed with the free-running laser.
The phase of the feedback signal can be controlled with a piezo mounted mirror located between the laser diode and the Fabry-Perot cavity. Optimum frequency noise reduction occurs when the free running laser frequency .omega..sub.0 is close to a cavity resonance .omega..sub.c and the system frequency .omega. is tuned with the feedback phase .phi. to be equal with .omega..sub.c. With a stable current supply and temperature control for the diode laser, the free running laser frequency can be made fairly stable so that stabilization of the feedback phase is the major problem in most experimental setups. A number of techniques have been employed to stabilize .phi. all using the condition .omega.=.omega..sub.c at the optimum locking point. Some methods use FM locking techniques. These include: a) modulation of the feedback optical path length producing changes of the laser frequency and analysis of the cavity response with lock in techniques or b) by using an FM sideband method in which the injection current is modulated at a high frequency so that modulation sidebands in the output spectrum are created. A detuning of the carrier frequency from the cavity resonance produces a phase shift of the carrier relative to the sidebands and an amplitude modulation in the reflected beam which when demodulated can provide a control signal.
In another technique, the polarization state of the light reflected from the cavity is analyzed while an additional polarizing element inside the cavity produces different complex field reflection coefficients for orthogonal polarizations of the incoming beam. The reflected light is linearly polarized if the frequency is on cavity resonance. Upon detuning, the reflected beam has a circularly polarized component with opposite rotation sense on each side of the resonance. The polarization character of the reflected beam can be detected and used to generate a control signal.
Modulating the system frequency has two disadvantages. First, the additional sidebands or frequency broadening in the laser frequency spectrum make it more complicated to analyze complex spectral data. Second, the laser spectral brightness (and power in the carder frequency) is reduced.
Polarization analysis techniques for feedback phase stabilization avoid these problems but the need of a polarizing element inside the locking resonator is disadvantageous; the intracavity polarizer must be aligned carefully, making it difficult or impossible to use a stable single block design for the Fabry-Perot cavity; with a perfect alignment, the polarizer will have some losses for the preferred polarization component. Therefore, with this technique it would not be possible to use a very high finesse (F.sub.c) resonator and the reachable noise reduction factor .eta. would be limited, since .eta. is proportional to 1/F.sub.c.
There are numerous United States patents that disclose optical feedback in semiconductor lasers. Many of these use a Fabry-Perot resonator, such as the one disclosed in the Kaminow U.S. Pat. No. 4,198,115. The following patents are typical: Hadley et al, U.S. Pat. No. 4,751,705 (optical injection in a single end-element of a semiconductor laser array to lock the array output); Smith et al U.S. Pat. No. 4,556,980 (injection locked semiconductor laser using a gas laser produces a narrow linewidth and frequency control); Haus et al U.S. Pat. No. 4,464,759 (semiconductor diode laser with microwave mode locking): Dutcher et al U.S. Pat. No. 4,752,931 (injection seeded Q switch laser with selectively introduced light from a master oscillator); Fujita et al U.S. Pat. No. 4,677,630 (frequency stabilization technique utilizing optical feedback of a fraction of the laser output); Beene et at. U.S. Pat. No. 4,606,031 (electrical-optical feedback to a transducer to change the resonant characteristics within the laser cavity) and, Dahmani et al. U.S. Pat. No. 4,907,237 (optically coupling a semiconductor laser to an external resonator having an optical cavity with a particular resonance frequency, including optical feedback).
The foregoing references illustrate current approaches to the solution of laser stabilization. However, these approaches do not offer the performance with respect to relative simplicity and general applications for frequency stabilization that the present invention provides.
The present invention not only provides a method and apparatus for stabilization using optical feedback to lock a laser to an external cavity, but as well it provides a method and apparatus of stabilization that allows the locking of an external cavity to a laser. A control signal that relates to the spatial pattern of excited transverse modes at the output of the external cavity can be used to control the resonance of the external cavity, or alternatively, can be used to control and stabilize the laser by adjusting an optical feedback signal.
As a particular embodiment of the method, a phase mirror control technique is provided in which a locking signal is generated by comparing the relative intensities of higher order transverse modes excited in a nearly-confocal, locking resonator. The transverse modes are not degenerate in frequency as in the confocal case. As a result, spatial mode patterns inside the cavity change when the system frequency is tuned over the cavity resonances, which occurs for example, if the phase mirror position is changed from the optimum locking point. These mode patterns can be observed in any beam transmitted through the cavity. The difference signal from two photodetectors sensing certain positions in the beam cross section can be used to lock the feedback phase to a point where the intensity of the light reflected back from the cavity into the diode laser reaches its maximum and the linewidth noise factor .eta. has a minimum.
The method for feedback phase stabilization does not require any dither on the system frequency or additional optical elements in or outside the locking cavity as is necessary in the stabilization techniques employed thus far. Besides leading to greater simplicity in the realization, this method offers the advantages of no imposed linewidth modulation or broadening, together with the possibility for using very high finesse locking cavities making the technique very suitable for high resolution spectroscopy.
It is an object of the invention, to provide a method for generating a control signal representative of the relative frequency between laser beam and an external resonant cavity.